Introduce an Olingo4 results-already-seen property to filter old results

The current polling implementation of the olingo4 consumer is to extract results from the target OData service every n seconds, with the results of each polling being identical. In the same way as the twitter component it would be useful for the consumer to recognise individual results as 'already seen' and filter them out. This has the following advantages:

• A set of polling results no longer contains the 'noise' of repeated results;
• With the possibility of all results being filtered out since they've already been seen, it is possible for the polling to idle and await new data rather than constantly returning the same results repeatedly.